Single phase motors, particularly those in the integral horsepower range, typically have lower performance and higher cost than three phase motors. Consequently, where three phase power is available, for applications requiring all but the smallest drive motors, three phase motors are typically used. However, many rural areas of the United States are supplied only with single phase AC power. Thus, farms and other agricultural operations which require electric motor drives have commonly been limited to using single phase motors.
Various solutions have been proposed for converting single phase power to three phase power to allow three phase motors to be utilized where only single phase power is available. Some single phase to three phase converters have been available commercially. Nonetheless, the relatively significant additional cost associated with the single phase to three phase converters has limited the widespread use of three phase motors in agricultural applications.
To minimize costs, it is desirable that the number of power switching elements in a single phase to three phase converter be minimized. Some prior converters which allowed for variable speed drive of induction motors utilized a full rectification of the single phase input power, with at least six switching devices in a full bridge inverter providing the three phase output power. While such systems are effective to provide three phase power to drive motors at variable speed, they have significant cost because of the number of switching devices required, which makes such systems economically impractical for many applications. Single phase to three phase converters have been proposed which utilize as few as two inverter switching elements to provide the third phase input to a three phase motor, with the two single phase input lines directly connected to the other two inputs to the motor. While such systems can be effective and economical, difficulties are encountered during starting of the motor, necessitating an additional large capacitor or capacitors to start the motor and additional switching elements to switch the capacitor out of the circuit after starting. Such two switch inverter circuits typically have not been capable of providing variable speed drive to the motor, but rather only provide balanced three phase output voltages at rated speed of the motor.